1. Field of the Invention
This invention relates to steel cords as a reinforcement for rubber articles such as tire, rubber crawler, belt and so on.
2 Related Art Statement
As the steel cord reinforcing the conventional rubber article such as heavy duty pneumatic radial tire, FIG. 1 illustrates a known steel cord 7 of 3+9+15 layer construction comprising a core 2 composed of three twisted filaments 1, an inner sheath 4 composed of nine filaments 3 spirally wound around the core 2 and an outer sheath 6 composed of fifteen filaments 5 spirally wound around the inner sheath 4. Moreover, numeral 8 is a spiral filament. In case of using the steel cord 7, the resistance to tread cut and the resistance to belt end separation are good, but the resistance to cut separation is somewhat a problem. This is because, the filaments 1, 3 and 5 constituting the core 2, inner sheath 4 and outer sheath 6 close to each other to thereby form a space obstructing the penetration of rubber into the inside of the steel cord 7. When this space is formed inside the steel cord 7, if cut failure is produced, for example, in the tread and water arrives at the steel cord through the cut failure, the water intrudes into the space to cause rust. Thus, a separation failure is caused at the boundary between the steel cord 7 and rubber.
In order to solve this problem, there has hitherto been proposed a steel cord as disclosed in U.S. Pat. No. 4,158,946 (corresponding to Japanese Patent laid open No. 54-50,640). In this steel cord, the number of filaments constituting the inner and outer sheaths is reduced to form minute gaps between filaments constituting the inner sheath and the outer sheath for penetrating rubber into the inner and outer sheaths.
Furthermore, Japanese Utility Model laid open No. 56-103,092 discloses a steel cord of 3+9+15 layer construction, wherein the diameter of the core is increased by 5.about.60% with respect to the filament diameter of each of the second and third layers.
Even in the latter steel cords, however, the resistance to cut separation could not sufficiently be enhanced because a triangular space obstructing the intrusion of rubber remains in the center of the core composed of three closed filaments. As a result, if water penetrates into this space, rust is produced to cause the separation failure as mentioned above.